Musannaf Ibn Abi Shayba 1:22

Zayd ibn al-Hubab narrated to us, he said: Amr ibn Abd Allah ibn Wahb al-Nakha'i narrated to me, from Zayd al-'Ammi, from Anas ibn Malik, from the Prophet (peace be upon him): "Whoever performs ablution and says: 'I bear witness that there is no god but Allah alone, He has no partner, and I bear witness that Muhammad is His servant and His messenger,' three times, the eight gates of Paradise will be opened for him; he may enter through whichever of them he wishes."